During the Spanish Civil War, huge numbers of boys left their homes and volunteered to fight against fascism.

Now thousands of angry young men and women are fighting a very modern war against ISIS without even leaving their bedrooms.

Mirror.co.uk has spent several weeks talking to a shadowy group of computer experts who have banded together under the name Varuna Group to do their bit in the conflict against the so-called Islamic State.

This Anonymous-inspired "virtual international brigade" agreed to tell their story in a bid to inspire others to join the battle against an enemy which expertly uses the internet to spread propaganda and win over new recruits.

We have agreed to withhold the real names of the group, which told us that most of their members are actually women.

Propaganda: A screenshot from an alleged ISIS supporter's account

"We know how effective ISIS is at using online media to recruit people," one member of the group said.

"It sickened me to see all the violent images and videos they put online and most especially how young people were somehow perversely attracted to them.

"When you see the violence that these ISIS members put out into the world, you really want to take them on."

Cyber-sects

Terror: An alleged jihadi bride shares a chilling warning

The Varuna Group showed us how one prominent female extremist online propagandist was banned from Twitter, but kept popping up again with a slightly different username to begin spreading the same vile message.

Her account relaunched yesterday using a chilling picture of a soldier manhandling a young Yazidi "child bride" as its banner image, and was quickly followed by more than 1,000 people before Twitter suspended it.

Even though this female extremists' account is up for hours at at time, it is used to spread horrifying videos, make vicious threats and share links to the so-called Encyclopedia of Terror.

Some of the most famous propagandists pop up hundreds of times using slightly different names, but using the same violent rhetoric.

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"We found manuals explaining how to make ricin or assassinate, kidnap, or perform lone wolf attacks in the West," one of the Varuna hacktivists told us, before sending links to the terrifying content.

"We found operational security files, showing how to keep your privacy online, and how to make sticky bombs [which can be attached to vehicles or other targets].

"The whole thing is perverted."

Twitter wars

(Photo: Getty / REUTERS)

If you know where to look, it is easy to find everything from bomb-making recipes to religious propaganda on Twitter, because it is next to impossible for the social network to monitor every tweet.

Cyber-jihadis have built up a huge amount of information, which is invaluable to terrorists and sleeper cells on a number of websites, as well as a vast library of execution videos.

Links to this content are then openly shared on the social network, which often relies on individuals to report dangerous content because of the sheer volume of messages posted every single day.

Hackers think it should do more to tackle the problem.

"American companies are letting sworn enemies of everything we stand for as a nation and a culture spew their culture of hate using their technology," one hacktivist said.

"Why on earth is a company like Twitter not doing anything about this?"

He called on Twitter to install image-recognition software to quickly identify extremist iconography and employ a full-time team to monitor content to make sure ISIS was not able to publish its dangerous propaganda.

Tactics

The Anonymous-linked hacktivists exclusively shared details of the anti-ISIS strategy, which is contained in the following four steps:

Sabotage ISIS's online image, which often entices young and impressionable people to join their ranks

Report imminent threats to the authorities

If there is no imminent threat or important intelligence present on the account, report ISIS accounts to Twitter and get them suspended.

Catalogue all other ISIS online profiles, hashtags, websites and other links in a comprehensive document to be shared with authorities

The group make a great effort to make sure their targets do not know they are being surveilled.

"We work very closely within our group and with others, especially when we need people to avoid reporting an account that contains an imminent threat," one female hacker said.

"In those cases, we do everything to keep those accounts alive as long as possible.

"There is a lot of work involved in communicating privately so the alleged terrorist is not tipped off."

Shell shock

(Photo: Reuters / Getty)

"We are volunteers doing it in our free time because we care," the hackers added.

"And that’s what it’s all about – doing whatever we can.

"We are only little drops into the huge ocean. But maybe our little drops make the difference. We want to encourage everyone to join in the hunt for Daesh ."

But part of the work of tackling ISIS online involves trawling through their propaganda material, which features beheadings and sickening videos of victims being burned alive, blown up or thrown from tall buildings.

"I cannot tell you how shocked I am to see what goes on here," the hacktivist continued.

"The violence is extreme and young kids see it too.

"But all this violence and cruelty only strengthen our resolve in the fight against Isis."